1. The reagent, not the substrate, sets substitution versus elimination.
Once you know the carbon, the species that attacks decides the fate of the reaction.
2. A strong, unhindered nucleophile that is a weak base gives SN2.
Cyanide, azide, thiolates, and iodide reach carbon faster than they grab a proton, so they substitute.
3. A strong, bulky base gives E2 and the less-substituted Hofmann alkene.
tert-Butoxide, LDA, and DBU are too hindered to reach carbon, so they pluck a proton instead.
4. A strong base that is also a good nucleophile leans E2 on 2°/3° substrates.
Hydroxide and ethoxide can substitute, but crowded carbons push them to eliminate.
5. A weak, neutral reagent can only do SN1/E1, and only if the substrate ionizes.
Water and alcohols are often just the solvent, so they wait for a carbocation to form.
6. Summary.
Strong Nu / weak base → SN2 · Strong bulky base → E2 (Hofmann) · Strong Nu-base on 2°/3° → E2 · Weak neutral reagent → SN1/E1.
Quiz yourself
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SN2. Azide is a strong, unhindered nucleophile but a weak base, so it substitutes.
E2 to the less-substituted (Hofmann) alkene, because the bulky base can't reach carbon and prefers the more accessible proton.
Water is a weak, neutral nucleophile/base, so it can't force a bimolecular step; it must wait for the substrate to ionize to a carbocation.
Mostly E2. Hydroxide is a strong base and decent nucleophile, but the crowded 3° carbon blocks backside attack, so elimination wins.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, wedge/dash bonds, and a clickable periodic table built in. No account needed.